Proton exchange membrane fuel cell with stepped channel bipolar plate

ABSTRACT

A fuel cell assembly includes a pair of corrugated bipolar plates. Each of the plates is defined by peak portions and sidewalls connecting the peak portions. The plates are fitted and nested within each other such that the sidewalls are in direct contact. Some of the sidewalls include a stepped shoulder portion such that each of the some of the sidewalls and the peak portions adjacent thereto form a stair-step profile and define a flow channel having a depth greater than a width.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.13/833,348, filed Mar. 15, 2013, which is a continuation-in-part ofapplication Ser. No. 13/593,562, filed Aug. 24, 2012, the disclosures ofeach of which are incorporated in their entirety by reference herein.

TECHNICAL FIELD

This disclosure relates to proton exchange membrane (PEM) fuel cells andto the construction and arrangement of bipolar plates therein.

BACKGROUND

A proton exchange membrane fuel cell is an electrochemical energyconversion device that converts hydrogen and oxygen into water, and inthe process produces electricity. Hydrogen fuel is channeled throughflow fields to an anode on one side of the fuel cell. Oxygen (from theair) is channeled through flow fields to a cathode on the other side ofthe fuel cell. At the anode, a catalyst causes the hydrogen to splitinto hydrogen ions and electrons. A polymer electrolyte membranedisposed between the anode and cathode allows the positively chargedions to pass through it to the cathode. The electrons travel through anexternal circuit to the cathode, which creates an electrical current. Atthe cathode, the hydrogen ions combine with the oxygen to form water,which flows out of the cell.

SUMMARY

A fuel cell assembly includes a metallic bipolar plate (MBPP) defining aseries of flat-bottom V portions of alternating orientation each havingsidewalls that include shoulders and a flattened vertex interconnectingthe sidewalls to form a stair-step flow channel having a maximum depthgreater than a maximum width to reduce material thinning and differencesin material strain across the MBPP. A surface defined by the flattenedvertex being coated or textured to be hydrophilic.

A fuel cell assembly includes a pair of corrugated bipolar plates, eachdefined by peak portions and sidewalls connecting the peak portions,fitted and nested within each other such that the sidewalls are indirect contact. Some of the sidewalls include a stepped shoulder portionsuch that each of the some of the sidewalls and the peak portionsadjacent thereto form a stair-step profile and define a flow channelhaving a depth greater than a width.

A fuel cell assembly includes a pair of metal bipolar plates (MBPPs),each defining a series of flat-bottom V portions of alternatingorientation and having sidewalls that include shoulders and a flattenedvertex interconnecting the sidewalls to form a stair-step flow channelhaving a maximum depth greater than a maximum width. The MBPPs arefitted and nested within each other with a same orientation such thatthe sidewalls are in direct contact. The assembly further includes amembrane electrode assembly in direct contact with one of the MBPPs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view of a bipolar plate flowchannel. Channel width is labeled with a “W” and channel depth islabeled with a “D.”

FIG. 2 is a diagrammatic cross-sectional view of a conventional bipolarplate flow channel having a trapezoidal shape.

FIG. 3 is a diagrammatic cross-sectional view of a fuel cell stackdisposed within a vehicle and including bipolar plates having flowchannels defined at least partially by stepped sidewalls.

FIG. 4 is a diagrammatic cross-sectional view of a bipolar plate havingflow channels at least partially defined by stepped sidewalls.

FIG. 5 is a diagrammatic cross-sectional view of a bipolar plate flowchannel. The channel depth is at least equal to the channel width. Likenumbered elements among the various figures can have similardescriptions.

FIG. 6 is a diagrammatic cross-sectional view of a bipolar plate flowchannel. The sidewalls each include two shoulder projections.

FIG. 7 is a diagrammatic cross-sectional view of portions of twoadjacent unit cells of a fuel cell stack. The bipolar plates are incontact with each other. One of the bipolar plates has flow channels atleast partially defined by stepped sidewalls. The other of the bipolarplates has flow channels which are trapezoidal in shape.

FIG. 8 is a diagrammatic cross-sectional view of portions of twoadjacent unit cells of a fuel cell stack including a centerplatedisposed between and in contact with bipolar plates of the fuel cells.

FIG. 9 is a diagrammatic cross-sectional view of portions of twoadjacent unit cells of a fuel cell stack. The bipolar plates are atleast partially nested with each other. One of the bipolar plates hasflow channels at least partially defined by stepped sidewalls. The otherof the bipolar plates has flow channels which are trapezoidal in shape.

FIG. 10 is a diagrammatic cross-sectional view of portions of twoadjacent unit cells of a fuel cell stack. The bipolar plates are atleast partially nested with each other. Both of the bipolar plates haveflow channels at least partially defined by stepped sidewalls.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill in the art will understand,various features illustrated and described with reference to any one ofthe figures can be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

Candidate metallic bipolar plate (MBPP) materials can be formed into aseries of channels having widths and depths designed to satisfy desiredfuel cell performance criteria. To increase fuel cell performance, deep,narrow channels with vertical side wall geometries essentially mimickinga flat bottom “U” are preferred in certain circumstances. Suchgeometries, however, can be difficult or impossible to form from thinmetallic materials in a cost effective manner. Formability limits ofcertain thin metallic materials, such as stainless steel foil, can thusrestrict their usage as MBPP materials for fuel cell applications. Forexample, stamping deep, straight channels into thin metallic materialscan produce excessive material thinning at channel geometry transitionregions such as at channel edges. Such thinning can result in tearing ofthe plate during channel formation, assembly of the fuel cell, oroperation of the fuel cell stack. Moreover, to the extent that thebipolar plate is a structural component of the fuel cell stack, suchthinning can compromise the rigidity of the bipolar plate.

Conventional MBPP designs commonly feature channels with cross-sectionsresembling a flat-bottom “V” (or trapezoidal shape). Theseconfigurations tend to have moderate side wall angles and restrictedchannel depths in an effort to accommodate the forming limits of theprecursor plate material and to minimize strain-induced thinning duringthe forming process. In some cases, base alloy processing steps can bealtered to improve the ability of MBPP precursor materials to form pasttheir normal limits. Alteration of the material base chemistry ormanufacturing process, however, can detrimentally impact othercharacteristics desired of an alloy to be used in fuel cell applicationssuch as corrosion resistance and electrical conductivity. Changes inmaterial composition and processing can also be cost prohibitive.

In fuel cells, increasing flow channel cross-sectional area,particularly on the cathode side of the respective membrane electrodeassembly (MEA), can substantially increase fuel cell performance. If thechannel opening is too wide, however, the MEA can bow inward toward thechannel. For this reason, it could be preferable for the channels to beformed with narrower openings and deeper channels.

The ability to form MBPPs with deeper channels, particularly when thechannels are formed by a stamping process, can be improved by alteringthe forming limits of the precursor plate material at the expense ofother characteristics as mentioned above. It has been discovered,however, that altering channel geometry to accommodate the inherentforming limits of the selected precursor material can also improve theability to form MBPPs with deeper channels without significantlyimpacting such characteristics as corrosion resistance and electricalconductivity. Disclosed herein are examples of “stepped” sidewall MBPPchannel geometries as shown, for example, in FIG. 1. Flow channels withstepped sidewalls can be distinguished from the more traditionaltrapezoidal channel configuration shown in FIG. 2.

The segments of the sidewall forming the shoulder (or step) need notform a 90 degree angle relative to each other. Any suitable angle (e.g.,80 degrees, 100 degrees, etc.) that permits deep channel formationwithout significant thinning can be used. Testing and/or simulation candetermine optimum step dimensions. Moreover, certain portions of thechannels may be made hydrophobic or hydrophilic via, for example,nano-texturing or coating to draw water away from reactant flow paths.

Finite element analysis (FEA) of the stepped sidewall geometry (shown,for example, in FIG. 1) has been compared to FEA of a traditionaltrapezoidal-shaped channel (shown, for example, in FIG. 2) withequivalent depth. This comparison revealed that material thinning of thestepped geometry of FIG. 1 is far less than that of the trapezoidalchannel geometry of FIG. 2, and material strain across the steppedsidewall geometry of FIG. 1 is more balanced. The FEA comparison alsorevealed that for the equivalent channel depth, D, the trapezoidalchannel of FIG. 2 is more likely to experience material failure in itshighly strained upper radius zones, R. The FEA model results have beenempirically verified in further studies. Usage of the stepped sidewallgeometry similar to that illustrated in FIG. 1 could allow for deeperchannels with greater sidewall angles, A, to be formed from existingmetallic materials while maintaining acceptable channel opening widthsW. These two characteristics can result in improved fuel cell stackoperational performance without diminishing the structural integrity ofinterfacing fuel cell stack components.

Referring to FIG. 3, a vehicle 98 such as a car can include a fuel cellstack 100 arranged, as known in the art, to provide power to move thevehicle 98. The fuel cell stack 100 can include a plurality of fuelcells 102 electrically connected together. Each of the fuel cells 102can include a membrane electrode assembly (MEA) 104 disposed betweenfirst and second bipolar plates 106, 108. The membrane electrodeassembly 104 includes a cathode portion on one side and an anode portionon the other side. Where the term “Gas” is used in the figures, it isintended to represent the fuel of the fuel cell 102 exposed to the anodeside of the MEA 104. In a hydrogen fuel cell, for example, the Gas wouldbe hydrogen gas. Where the term “OX” is used in the figures, it isintended to represent oxygen (or air containing oxygen) exposed to thecathode side of the MEA 104.

Referring to FIG. 4, each of the bipolar plates 106 can be stamp-formedfrom a precursor metal sheet such as a sheet of stainless steel foil orother appropriate conductive metallic material. Alternative formingmethods such as hydro-forming and adiabatic forming can also be used.Each of the bipolar plates 106 defines adjacently aligned flow channels110 (normal to the page) alternately disposed on opposing sides of thebipolar plate 106. Further, each of the bipolar plates 106 includes atleast partially stepped sidewalls 112 having shoulder portions 114, andproximal and distal peak portions 116, 118 where the stepped sidewalls112 connect with each other (giving the bipolar plate 106 a corrugatedappearance). Hence, each of the stepped sidewalls 112, in this example,have two end portions and a body portion disposed between the endportions. Each of the end portions is adjacent to one of the peakportion 116, 118. The shoulder portions 114 are formed in the bodyportions. The proximal peak portions 116 of each bipolar plate 106 canbe in direct contact with the MEA 104 (FIG. 3). The distal peak portions118 of adjacent bipolar plates can be aligned and in electrical contactwith one another.

Particularly in instances in which the bipolar plates 106 arestamp-formed, the bipolar plates 106 can have a substantially uniformweb thickness, T. Such thickness can be, for example, in the range ofapproximately 100 microns. Any suitable thickness, however, can be used(e.g., 80 to 250 microns, etc.) A similar description applies to thebipolar plates 108 of FIG. 3.

Referring to FIG. 5, a portion of a bipolar plate 206 includes at leastpartially stepped sidewalls 212 having shoulder portions 214 andproximal and distal peak portions 216, 218 respectively. The channeldepth, D, in this example, is at least as equal to the channel width, W.In other examples, the channel depth, D, can be greater than the channelwidth, W. For example, D can be approximately 500 microns and W can beapproximately 100 microns.

A surface 219 of the distal peak portion 218 is nano-textured so as tomake it hydrophilic. Appropriate coatings may also be used to achievethis effect. The trough formed by the stepped sidewalls 212 and thehydrophilic surface 219 act to draw in water and keep it out of reactantflow paths. Other surfaces of this, and other embodiments, can of coursebe made hydrophilic or hydrophobic. In one example, surfaces betweenproximal and distal peak portions 216, 218, and surrounding the surface219 are made hydrophobic to further direct water toward the troughformed by the stepped sidewalls 212 and the surface 219. In anotherexample, all surfaces are made hydrophilic, etc.

Referring to FIG. 6, a portion of a bipolar plate 306 includes at leastpartially stepped sidewalls 312 having shoulder portions 314 andproximal and distal peak portions 316, 318 respectively. In thisexample, each of the stepped sidewalls 312 can have two (or more)shoulder portions 114. Other configurations are also contemplated.

Referring to FIG. 7, a portion of a fuel cell stack 400 includes MEAs404 and bipolar plates 408, 420 in contact with each other and disposedbetween the MEAs 404. In this example, the bipolar plate 408 includesstepped sidewalls 412 and the bipolar plate 420 does not.

Referring to FIG. 8, a portion of a fuel cell stack 500 includes MEAs504, bipolar plates 506, 508, and a center plate 522. The center plate522 is disposed between and in contact with the bipolar plates 506, 508to prevent nesting of adjacent bipolar plates and to increase the numberof coolant flow channels associated with the bipolar plates 506, 508.Other arrangements are also contemplated.

Referring to FIG. 9, a portion of a fuel cell stack 600 includes MEAs604 and bipolar plates 608, 620. Similar to the example of FIG. 7, thebipolar plate 608 includes stepped sidewalls 612 and the bipolar plate620 does not. Additionally, the bipolar plate 608 in this exampleincludes textured or coated hydrophilic portions 619. The bipolar plates608, 620 are arranged such that their sidewalls are in contact with eachother (e.g., connected via welding, bonding, etc.), which can increasesurface contact (and electrical conductivity) therebetween, providealternative weld locations, and decrease stack height.

Referring to FIG. 10, a portion of a fuel cell stack 700 includes MEAs705 and bipolar plates 706, 708. Similar to the example of FIG. 9, thebipolar plates 706, 708 are arranged such that their sidewalls are incontact with each other to form a nested pair.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments may havebeen described as providing advantages or being preferred over otherembodiments or prior art implementations with respect to one or moredesired characteristics, those of ordinary skill in the art recognizethat one or more features or characteristics can be compromised toachieve desired overall system attributes, which depend on the specificapplication and implementation. These attributes can include, but arenot limited to: cost, strength, durability, life cycle cost,marketability, appearance, packaging, size, serviceability, weight,manufacturability, ease of assembly, etc. As such, embodiments describedas less desirable than other embodiments or prior art implementationswith respect to one or more characteristics are not outside the scope ofthe disclosure and can be desirable for particular applications.

What is claimed is:
 1. A fuel cell assembly comprising: a metallic bipolar plate (MBPP) defining a series of flat-bottom V portions of alternating orientation each having opposite sidewalls that include shoulders: and a flattened vertex interconnecting the sidewalls to form a stair-step flow channel having a maximum depth greater than a maximum width to reduce material thinning and differences in material strain across the MBPP, a surface defined by the flattened vertex being coated to be hydrophilic; and a second MRPP, having a same size and shape as the MBPP, fitted and nested within the MBPP such that the MBPP and the second MBPP have same orientation and respective sidewalls thereof are in direct contact with each other such that a plurality of channels is formed between the MBPP and the second MBPP.
 2. The fuel cell assembly of claim 1, wherein other surfaces of the MBPP are textured or coated to be hydrophobic.
 3. The fuel cell assembly of claim 1, wherein the MBPP has a generally uniform thickness.
 4. A fuel cell assembly comprising: a metallic bipolar plate (MBPP) defining a series of flat-bottom V portions of alternating orientation each having opposite sidewalls that include shoulders, and a flattened vertex interconnecting the sidewalls to form a stair-step flow channel having a maximum depth greater than a maximum width to reduce material thinning and differences in material strain across the MBPP, a surface defined by the flattened vertex being textured to be hydrophilic; and a second MBPP, having a same size and shape as the MBPP, fitted and nested within the MBPP such that the MBPP and the second MBPP have same orientation and respective sidewalls thereof are in direct contact with each other such that a plurality of channels is formed between the MBPP and the second MBPP.
 5. The fuel cell assembly of claim 4, wherein other surfaces of the MBPP are textured or coated to be hydrophobic.
 6. The fuel cell assembly of claim 4, wherein the MBPP has a generally uniform thickness. 